The field of vascular tissue engineering seeks to generate functional blood vessels with properties similar to that of healthy tissue. Engineered tissues may originate from biocompatible scaffolds embedded with diverse populations of cells or through altering existing tissues exposed to controlled stimulation that initiate known intrinsic adaptive processes. The demand for such functional tissues is especially great in the vascular system, where engineered blood vessels (EBVs) could be used to replace diseased or damaged blood vessels in patients suffering from advanced stage atherosclerosis or other focalized degenerative diseases. For example, nearly 400,000 coronary artery bypass grafts and around 50,000 peripheral vascular grafts were performed in the United States each year. However, patients often lack viable autograft tissue and purely synthetic replacements (e.g., DACRON® grafts) become occluded when used to replace small diameter vessels. As such, there is a need for the development of functional EBVs that are biocompatible, are anti-thrombogenic, and that exhibit autograft-like levels of burst and fatigue strength. Accordingly, various approaches have been employed over the years to engineer blood vessels that meet these requirements, yet few have reached advanced stage clinical trials. Therefore the practical and commercial development of this technology remains an emerging field.
In the development of EBVs, the choice of scaffold material, cell type, and assembly are typically considered. The mechanical environment and the bioreactor utilized for culturing the EBV must also be considered. The mechanical environment includes the biaxial stresses and stretches generated in the circumferential and the axial directions of the EBV, as well as the fluid-induced wall shear stresses focused along the endothelial cell (EC) lined lumen of the EBV. In vivo, the circumferential loading is a result of pressurization and the axial loading is a result of somatic growth. It has also been observed that natural tissues seek to restore levels of mechanical stress. For example, in sustained hypertension, which elicits an acute increase in circumferential wall stress, the primary remodeling outcome is an increase in wall thickness, which in turn acts to restore the wall stress to baseline, or normotensive, stress levels. Accordingly, the mechanical environment has been identified as a major contributor to the growth and remodeling of a biomimetic EBV and is an important physical factor in vascular graft generation and homeostasis. Specifically, cyclic stretching of intramural vascular cells initiates proliferation, promotes the release of growth factors, alters fiber realignment, regulates smooth muscle cell (SMC) contractile phenotype, and encourages overall extracellular matrix (ECM) synthesis (e.g., collagen, tropoelastin, etc.) and tissue turnover by SMCs and fibroblasts. Similarly, the frequency, direction, and magnitude of shear stress on ECs governs metabolic activities, nutrient exchange, cellular morphology, stress fiber alignment, and SMC phenotype, and also control paracrine factors including nitric oxide release, which is a major mediator of remodeling and homeostasis.
Over the last fifty years, vascular perfusion bioreactors have been developed primarily as research tools but have recently become available in the commercial market. However, commercially available bioreactors are not designed to implement the comprehensive mechanical objectives discussed above that are needed to create a truly biomimetic EBV. As such, a need exists for a EBV bioreactor that has the capability to optimize mechanical (stress) objectives, which would, in turn, minimize culture time and maximize output. Furthermore, a need exists for a bioreactor specific for culturing EBVs that can impose and test biaxial loads, can deliver specific biomimetic pressure and flow profiles, can be scalable for different vessels and animals, and can provide for real time data collection and assessment. A need also exists for a bioreactor that can be autoclaved, maintain sterility for prolonged culture times, promote nutrient and gas exchange, actively maintain temperature and pH, permit cell seeding, and allow for chemical stimulation and assessment. Such a device could be used in the commercial setting and not solely as a research tool.
Moreover, the ultimate success of an EBV lies in its ability to perform in the intended environment. The biomimetic hemodynamic loads on the EBV are unique amongst species and anatomical location, thus the knowledge and application of vessel-specific hemodynamics and axial loading that mimic the intended graft condition are both crucial during tissue development in order to avoid hemodynamically-induced pathologies. As such, a need exists for a bioreactor that could prescribe dynamic pressure and flow waveforms during culture that incorporate complex phasic relationships that are not static or simply sinusoidal. In other words, a need exists for a bioreactor that can deliver variable stresses to an engineered blood vessel through the application and control of dynamic pressure and flow waveforms so that the engineered blood vessel can be conditioned and remodeled during ex-vivo culture so that it ultimately possesses properties that mimic a native blood vessel.